Note: my laptop screen is always closed while driving unless I'm using GPS.
Since the beginning (April 2005) I have used a Compaq Presario 2200 with a 1.3 GHz mobile processor and 512MB RAM in the car. I made a mount for it with a $40 center console and another $40 in plastic sheeting and miscellaneous hardware.
UPDATE 6-17-2007: I now have an HP dv6365 laptop, 1.85GHz Intel Core Duo T5600 with 2GB RAM and Vista. More info on the new setup can be found here.
Access database that tracks all fares with to/from, date, time, fare, and whether it was a flag or bell. It was my first time using Access, and considering there is only one table, it was easy. I wrote a few basic queries, like daily income, monthly income, annual income, etc. Many more are in the works.
GPS navigation with Microsoft Streets & Trips. I purchased S&T as a package with the GPS receiver, which plugs into the laptop via USB. It was about $110 at Best Buy. It acts like an in-dash GPS system, with a car icon on the map, and voice directions. Every address in the U.S. and Canada is included, and there is a library of hotels and places of interest. The S&T interface includes a compass and speedometer.
GPS receiver sits on dash
Internet with cellular (EVDO) service. I use a PC card for a dedicated, unlimited internet connection anywhere I go -- service is $60 per month. It's the perfect complement to GPS. For instance, if a customer inquires about a sushi restaurant in a certain area, I can check the web, and if there is one, click on "map", or dump the address into Streets & Trips. I also use my cell phone to make reservations for customers. This is a customer service business, after all. Not all cabbies are surly bastards who learned to drive in Cairo and prefer grunting to the Queen's English.
I also surf the web for entertainment during slow times, including updating this blog. Like a lot of people, I touch on dozens of newspapers and news sources, worldwide, every week. The main ones I hit every day are BBC, Google News, Yahoo News, and the San Diego Union-Tribune. I also enjoy reading blogs, magazines and tech sites.
Games, like Sim City IV
A laptop is the ultimate iPod. I connect the audio-out jack to the auxiliary jack on the front of a recently purchased Kenwood deck. This is vastly superior to an FM transmitter, which I used for a while. I have a lot of MP3s sitting on an external hard drive at home, and I transfer files to the laptop via USB. Also, I have a flash drive for transfers. Flash is good to swap music and video files among drivers, too -- several other drivers got laptops after seeing my setup.
Card reader and flash drive -- uses SD cards from my camera
DVDs and downloaded movies, and recorded TV shows. I rent DVDs, and check them out from libraries, and I use a movie download service, Movielink, for renting movies directly onto the hard drive. At home I have a USB TV-in box, which lets me record cable TV shows to the HD; I watch them the following day while sitting on cab stands. Audio runs through the car stereo, just like the music.
I keep a 5mp, 12x opti-zoom digital camera with me in the car. I transfer photos directly to the laptop, and use Photoshop to edit.